The present invention relates to a blue color filter that includes a dye or a pigment. The present invention also relates to a blue color filter that is typically used in combination with an organic electroluminescent element (hereinafter referred to as an xe2x80x9cEL elementxe2x80x9d).
Research and development of various display devices have been vigorously explored to meet the increasing demand for flat panel displays to replace conventional cathode ray tubes (CRT""s). EL elements have been developed as all-solid-state and self-light-emitting devices to meet this demand. The EL elements have attracted much attention due to their high definition and high visibility, which the other display devices are unable to attain.
Various measures have been proposed for providing flat panel displays with polychromatic-display function or full-color-display function. Japanese Unexamined Laid Open Patent Applications S57-157487, S58-147989 and H03-214593 disclose flat panel displays that arrange light emitters for the three primary colors, i.e. green-light emitters, blue-light emitters and red-light emitters, in a matrix. The light emitters for the respective colors emit light independently. In order to apply an organic EL element to this type of flat panel display, fine and precise arrangement of the three kinds of light emitting materials is required in the matrix for each of the three primary colors. However, it is difficult to obtain fine and precise arrangement of the three kinds of light emitting materials in a matrix without increasing manufacturing costs. In addition, color deviations occur over time because the lives of the three kinds of light emitters are different from one another. The x-value and the y-value on the Commission Internationale de l""Eclairage (CIE) color coordinates for the reported blue organic EL device are 0.16 and 0.15, respectively (cf. Japanese Unexamined Laid Open Patent Application H08-286033). Comparing the CIE color coordinates for the reported blue organic EL device to the NTSC hue (that is a hue of standard blue for CRT""s corresponding to x=0.14 and y=0.08 on the CIE color coordinate scale), reveals that the blue color from the reported blue organic EL device is not pure enough.
Japanese Unexamined Laid Open Patent Applications H01-315988, H02-273496 and H03-194895 disclose flat panel displays which dispose color filters on a white back light source so that the three primary colors may be obtained through the color filters. This configuration requires white light with high luminance in order to obtain the three primary colors with high luminance. However, an organic EL element capable of emitting white light with high luminance for long durations has not yet been obtained.
Japanese Unexamined Laid Open Patent Application H03-152897 discloses a color conversion arrangement where fluorescent materials are arranged two-dimensionally and separately convert the light from a light emitter to fluorescent light of desired colors. This color conversion arrangement is applied to CRT""s and flat panel displays. Japanese Unexamined Laid Open Patent Applications H03-152897 and H05-258860 disclose a color conversion method that uses fluorescent materials for color filters. The flourescent materials absorb light in the range of light emitted by an organic EL element and then emit visible fluorescent light. An organic EL element that emits brighter light than visible light is more suitable as the light source since the organic EL element should emit light that is not limited to white. Japanese Unexamined Laid Open Patent Applications H03-152897, H08-286033 and H09-208944 disclose color conversion arrangements that use a blue-light-emitting organic EL element and convert the blue light to the wavelengths (colors) of green and red. A full-color light-emitting-type display that uses light with low energy such as near ultraviolet light and visible light is constructed by finely and precisely patterning a fluorescent light conversion film that contains fluorescent pigments.
Color filters used in combination with organic EL elements include a matrix of pixels for the three primary colors or a black matrix. The matrix is formed on a glass substrate by a dying method, a printing method, a pigment dispersion method or other similar methods.
In the dying method, a coloring material is made by dying a natural photosensitive resin such as gelatin, or a synthetic photosensitive resin such as aminated poly(vinyl alcohol) with an acidic dye. A color filter is obtained by coating the coloring material onto a glass substrate (cf. Japanese Examined Patent Application H01-5273). The coating film obtained by the dying method has problems with weather resistance, thermal resistance and moisture resistance.
In the printing method, a color filter is obtained by using a printing ink made by dispersing a pigment in a thermosetting resin or in an ultraviolet-ray-curing resin (cf. Japanese Unexamined Laid Open Patent Applications S62-54774 and S63-129303). The flatness and smoothness of the printed ink film surface pose certain problems and interfere with precise positioning of the color filter patterns for the three primary colors.
In the pigment dispersion method, small grains of 1 xcexcm or less in diameter of a red pigment, a blue pigment or a green pigment are dispersed in a photosensitive resin solution. The solution is then coated onto a glass substrate. The resulting photosensitive resin film is patterned by photolithography to form a desired pixel pattern (cf. Japanese Examined Patent Application H04-37987 and H04-39041).
Color filters made by the pigment dispersion method are used mainly to achieve the thermal resistance required for making color displays, the weather resistance required for using the displays and to the precision and fineness required for displaying images.
Color displays today must possess fine and precise pixel arrangement, display images with full colors and reduce power consumption. Most of the demand for electric power in operating the conventional color displays is for the back light. To meet these demands, it is necessary to improve the color purity, the chroma and the amount of light transmitted through the color filter. In order to increase the amount of light transmitted through the color filter, the amount of pigment in the photosensitive resin or the thickness of the film of pixels has been reduced.
However, the methods for increasing light transmission through the color filter reduce the chroma of the color filter. The reduced chroma impairs the brightness of the colors to be displayed and causes whitened images including the background. When the coloring pigment content is increased so as not to impair the chroma of the color filter, displayed images including the background are darkened. When the light intensity of the back light is increased to maintain brightness of the displayed images as well as the background, electric power consumption of the display is increased.
To improve the amount of light transmission, the diameter of the dispersed pigment particles are reduced to less than xc2xd of the wavelength of the color that the pigment exhibits (Kiyoshi Hasizume, JOURNAL of THE JAPAN SOCIETY of COLOR MATERIAL, December 1967, p. 608). Since the wavelength of the color that the blue pigment exhibits is shorter than the wavelengths of the colors which the green and red pigments exhibit, the blue pigment should be pulverized and dispersed more finely than the red and green pigments. This will increase manufacturing costs. In addition, stability of the dispersed pigment particles is reduced when the particles are too finely pulverized, posing a further problem.
Copper phthalocyanine blue having a crystal form of xcex1-type, xcex2-type or xe2x80x94type has been used widely as a blue pigment (HANDBOOK of COLOR MATERIAL ENGINEERING, p. 333, edited by THE JAPAN SOCIETY of COLOR MATERIAL). When xcex1-type copper phthalocyanine blue is used alone as a blue pigment in the color filter, its tinting strength is not very strong. A lot of pigment must be mixed into the photosensitive resin in order to obtain the desired chroma. But then, problems with thermal discoloration of the formed color filter and adhesiveness of the color filter and the glass substrate occur. Moreover, the amount of light transmission at wavelengths greater than 600 nm is excessive, impairing color purity.
When copper phthalocyanine blue of xe2x80x94type is used alone, less is required as compared to using xcex1-type copper phthalocyanine blue. The amount of xe2x80x94type needed to be added to the photosensitive resin is less because of its excellent tinting strength. However, as the mixing ratio of the pigment is increased to obtain the desired chroma, more shading is caused at the 365 nm wavelength. This interferes with effective curing of the photosensitive resin and results in reduced photo-curing sensitivity. This reduction in photo-curing sensitivity causes a reduction of the thickness of the developed film and distorts the pattern of the pixels.
When xcex2-type copper phthalocyanine blue is used alone, a large deviation from the desired NTSC hue is caused because xcex2-type copper phthalocyanine blue is greenish blue.
A color filter using a pigment mixture containing copper phthalocyanine blue and dioxazine violet has been proposed (cf. Japanese Examined Patent Application H06-95211, Japanese Unexamined Laid Open Patent Application H01-200353 and Japanese Examined Patent Application H04-37987). By using a color mixture of any of the three kinds of copper phthalocyanine blue and I. C. Pigment Violet 23, a kind of dioxazine violet is produced, light transmission between 500 and 550 nm is suppressed and the color purity is improved. However, light transmission in the desired blue range between 420 and 500 nm is also suppressed, causing insufficient brightness of the displayed images including the background. In actual operation of the display, use of a polarizer plate results in a reduction in light transmission for the desired blue range of from 70 to 80% compared to the light transmission for the other color ranges. Therefore, it has been necessary to increase the amount of light transmission through the blue color filter.
In light of the above, it is an object of the invention to provide a blue color filter having high transmittance of blue color and low transmittance of green color.
It is another object of the invention to provide an organic electroluminescent device that exhibits excellent and pure blue color.
Briefly stated, the present invention is a blue color filter including a photosensitive resin and a dye containing at least a first cyanine dye described by the following structural formula (I): 
An embodiment of the present invention is a blue color filter including a photosensitive resin and a pigment containing at least a first cyanine dye described by the aforementioned structural formula (I).
Advantageously, the blue color filter contains from 0.5 to 30 weight parts of the first cyanine dye with respect to 100 weight parts of the photosensitive resin.
Advantageously, the dye containing the first cyanine dye further contains a second cyanine dye described by the following structural formula (II): 
Advantageously, the content of the second cyanine dye is from 0.5 to 1.5 weight parts with respect to 1 weight part of the first cyanine dye.
According to another embodiment of the invention, a blue color filter is provided including: a photosensitive resin; and a pigment containing at least a first cyanine dye described by the foregoing structural formula (I).
Advantageously, the pigment containing the first cyanine dye further contains copper phthalocyanine blue.
According to still another embodiment of the invention, an organic electroluminescent device is provided including: any of the blue color filters described above and an organic electroluminescent element laminated onto the blue color filter.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.